RNA editing can profoundly affect gene expression by altering the coding capacity of a transcript. RNA editing has been reported in transcripts from such organisms as trypanosomes, higher plants, and mammals. In humans, RNA editing is necessary for proper expression of certain genes in the brain and intestine. This process is responsible for the production of two different apolipoproteins from the same gene; as the abundance of one of these proteins is correlated with coronary artery disease, aberrant RNA editing could potentially have serious health consequences. It is likely that additional examples of RNA editing of human gene transcripts remain to be discovered. We propose to study RNA editing in plant organelles as a model for understanding this fundamental process that can control gene expression. Like mammals, plant organelle editing results in the modification of nucleotides rather than the insertion of nucleotides that occurs in trypanosomes and other organisms. The editing activity in chloroplasts and mitochondria may share features or even be identical. A cryptic higher-level genetic code may be specifying which nucleotides are to be altered by the editing process. Structural requirements for RNA editing will be examined in vivo by introduction of chloroplast and mitochondrial RNAs into transiently and stably transformed chloroplasts obtained by particle bombardment. After information about RNA substrate is available, studies will be carried out to characterize the components of the editing activity. To learn whether there is a mechanism that prevents synthesis of proteins from incompletely edited transcripts, or whether translation of unedited RNAs results in multiple variant proteins, we will isolate two mitochondrial proteins and characterize them thoroughly. To determine whether editing has a role in gene regulation, we will perform experiments to determine whether editing of particular genes is tissue-specific.